Transcriptional regulation represents the major mechanism of controlling gene expression, yet we have little direct knowledge of how genes are regulated in the whole body, due largely to an inability to observe and measure changes in gene expression under real physiological conditions and in real time. For similar reasons, it has not been possible to study directly the effects of given cancer therapies on their target(s), if mediated at the transcriptional level. However, the recent revolution in molecular imaging has yielded novel tools for performing noninvasive, in vivo imaging of gene expression. A biologically relevant application of imaging technology is to target, by homologous recombination, a reporter gene into the genomic locus of an endogenous gene so that the regulated expression of that gene can be monitored non-invasively, in real time, and in response to internal and external signals. We have established such a system using the mouse mdr1a locus as a proof of principle and as a biologically important gene. Multidrug resistance (MDR) remains a serious impediment to curative chemotherapy in cancer patients. One mechanism of MDR is the enhanced expression of the MDR1 gene. MDR1 overexpression has been associated with drug resistance in many human cancers, but its contribution to clinical outcomes remains unresolved. Systematic longitudinal studies to determine MDR1's role in resistance are difficult, if not impossible, to perform in humans and an adequate animal model to study such questions has not previously been available. The role of MDR1 in drug pharmacokinetics is well established, but the regulation of MDR1 in normal organs involved in drug uptake and clearance is poorly understood. In our preliminary studies: 1) We have engineered mdr1a+/fLUC mice that have firefly luciferase (fLUC) targeted to the mdr1a gene's genomic locus in a way that makes in-frame expression of fLUC conditional on Cre-mediated recombination. 2) We have shown that expression of fLUC under the control of the endogenous mdr1a gene locus is a faithful in vivo reporter for mdr1a expression in the basal, steady state. 3) We have also demonstrated that fLUC can be used to monitor induction of mdr1a gene expression in response to xenobiotic stimuli. We now propose to use this non-invasive model system to study mdr1a gene expression in the in vivo setting. We will also extend the model to be able to study both the cis-acting and trans-acting factors that control mdr1a gene expression at the transcriptional, post-transcriptional and translational level. Aim 1 is to determine if mdr1a is induced in normal organs in a tissue-specific fashion. Aim 2 is to determine if mdr1a is induced during breast cancer progression and/or treatment. Aim 3 is to determine if specifi trans-acting and cis-acting factors are required for mdr1a induction in specific tissues.